1. Field of the Invention
This invention relates to phosphorus-vanadium mixed oxide oxidation catalysts. More particularly, this invention relates to phosphorus-vanadium mixed oxide catalysts useful for the partial oxidation of non-aromatic hydrocarbons in the vapor phase with molecular oxygen or a molecular oxygen-containing gas to produce maleic anhydride in excellent yields, the catalyst exhibiting a single pass weight/weight productivity of at least 70 grams of maleic anhydride per kilogram of catalyst per hour.
Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these varied needs.
2. Description of the Prior Art
Numerous catalysts containing mixed oxides of phosphorus and vanadium are disclosed in the prior art as being useful for the conversion of various organic feedstocks to maleic anhydride, and further that such catalysts wherein the valence of the vanadium is between about +3.8 and +4.8 are particularly well suited for the production of maleic anhydride from saturated hydrocarbons having at least four carbon atoms in a straight chain. In many instances, these catalysts also contain added promoter elements which are considered to exist in the catalysts as the oxide. Common organic feedstocks include non-aromatic hydrocarbons such as n-butane, 1- and 2-butenes, 1,3-butadiene, or mixtures thereof.
Procedures for the preparation of catalysts containing the mixed oxides of phosphorus and vanadium are also disclosed and taught by the prior art. Many of such procedures teach that it is preferable to reduce the vanadium in solution to the tetravalent state. For example, these catalysts can be prepared by contacting phosphorus compounds and vanadium compounds under conditions sufficient to produce the tetravalent vanadium and to form the catalyst precursor. The catalyst precursor is thereafter recovered, dried, and calcined to produce the active catalyst.
U.S. Pat. No. 4,333,853 discloses a phosphorus-vanadium mixed oxide catalyst prepared by reducing vanadium substantially in the pentavalent valence state to a tetravalent valence state in the presence of a phosphorus-containing compound and in the absence of a corrosive reducing agent in an organic liquid medium capable of reducing the vanadium to a valence state less than +5, recovering the resulting vanadium-phosphorus mixed oxide catalyst precursor, drying such precursor, and calcining the precursor to obtain the active catalyst. Such catalysts reportedly are effective in the oxidation of C.sub.4 hydrocarbons such as n-butane, n-butenes (1- and 2-butenes), 1,3-butadiene, or mixtures thereof to produce maleic anhydride with selectivities ranging from 58.7% to 68.1% and yields (mole %) ranging from 51.4% to 59.5%.
U.S. Pat. No. 4,315,864 relates to a process for the production of maleic anhydride from C.sub.4 hydrocarbons in the presence of a phosphorus-vanadium mixed oxide catalyst. The catalyst is prepared by reducing a pentavalent vanadium-containing compound in an olefinic, oxygenated organic liquid medium to a +4 valence in the absence of a corrosive reducing agent, recovering the resultant catalyst precursor, drying the catalyst precursor, and calcining the precursor to obtain the active catalyst.
U.S. Pat. No. 4,312,787 describes a catalyst which comprises an inert support and a catalytically active mixed oxide material coating of phosphorus and vanadium or of phosphorus, vanadium, and uranium on the outer surface of the support in an amount greater than 50% to about 80% by weight of the combined support and oxide material. Catalysts within the scope of the claims of the patent were reported to produce maleic anhydride from n-butane in yields ranging from 53% to 62.5%, with selectivities ranging from 57.4% to 67.9%.
U.S. Pat. No. 4,294,722 discloses a process for preparing catalysts containing mixed oxides of phosphorus and vanadium. In this process, a pentavalent vanadium-containing compound is reduced (at least in part) to a +4 valence state in an organic liquid medium in which the vanadium compound is at least partially soluble to form a solution or mixture. Any unsolubilized vanadium-containing compound having a particle size greater than 0.1 mm diameter is removed. The resulting solution is mixed with a pentavalent phosphorus-containing compound to form a precipitate which is recovered, dried, and calcined. Such cataysts are reported to be effective in the oxidation of non-branched C.sub.4 hydrocarbons, such as n-butane, 1- and 2-butenes, 1,3-butadiene, and mixtures thereof, in the presence of molecular oxygen or a molecular oxygen-containing gas in the vapor phase to maleic anhydride with good selectivity.
U.S. Pat. No. 4,293,498 discloses a process for preparing phosphorus-vanadium mixed oxide catalysts. In this process, a pentavalent vanadium-containing compound is reduced to a +4 valence state in an olefinic, halogenated organic liquid-containing medium either in the presence or absence of a phosphorus-containing compound. The resulting catalyst precursor is recovered, dried, and calcined to produce the active catalyst. Such catalysts reportedly are effective in the oxidation of non-branched C.sub.4 hydrocarbons, such as n-butane, 1- and 2-butenes, 1,3-butadiene, and mixtures thereof, in the presence of molecular oxygen or a molecular oxygen-containing gas in the vapor phase to produce maleic anhydride in high yields and good selectivities.
In U.S. Pat. No. 4,187,235, a process is described for preparing maleic anhydride from n-butane in the presence of a phosphorus-vanadium oxygen high surface area catalyst, that is, 10 to 100 square meters per gram (BET method). The catalyst is prepared by reducing pentavalent vanadium to a valence between +4.0 and +4.6 with a substantially anhydrous primary or secondary alcohol and contacting the reduced vanadium with phosphoric acid, followed by recovering and calcining the resulting vanadium (IV) phosphate compound.
In U.S. Pat. No. 4,251,390, a zinc-promoted phosphorus-vanadium oxygen catalyst is disclosed and claimed. The catalyst is prepared by reducing pentavalent vanadium in a substantially anhydrous organic medium to a lower valence state and digesting the reduced vanadium in the presence of a zinc promoter compound. The resulting catalyst is activated by bringing the catalyst to operating temperatures for the oxidation of n-butane to maleic anhydride at a rate of 5.degree. to 10.degree. C. per hour in the presence of a butane-in-air mixture.
U.S. Pat. No. 4,132,670 discloses a process for preparing a crystalline vanadium (IV) phosphate catalyst composition having a surface area in excess of 10 square meters per gram. In this process, orthophosphoric acid is reacted with a vanadium (IV) oxycompound by contacting a suspension of the vanadium compound in a hydroxylic organic medium, for example, isobutyl alcohol, with the phosphoric acid at a temperature in the range of 20.degree. C. and 210.degree. C. until the conversion is completed. The resulting catalyst is activated by heating at elevated temperatures in a butane-in-air mixture.
U.S. Pat. No. 3,864,280 discloses phosphorus-vanadium mixed oxide catalysts having an intrinsic surface area from about 7 to about 50 square meters per gram. The catalysts are prepared by precipitation of a phosphorus-vanadium-oxygen complex from an essentially organic solvent medium in the absence of gross amounts of water. The resulting crystalline precipitate is activated by heating in air followed by a 1.5 mole percent butane-in-air mixture, both at elevated temperatures.
Although these prior art catalysts are effective to provide the desired product, maleic anhydride, the commercial utility of a catalyst system is highly dependent upon the cost of the system, the conversion of the reactant(s), and the yield of the desired product, or stated differently, the actual productivity of the catalyst system. In many instances, a reduction in the cost of a catalyst system on the order of a few cents per kilogram or pound, a small percent increase in the yield of the desired product, relative to the amount of catalyst required, represents a tremendous commercial economical savings and advantage. Accordingly, research efforts are continually being made to define new or improved catalyst systems and methods and processes of making new and old catalyst systems to reduce the cost and/or upgrade the activity, selectivity, and/or productivity of such catalyst systems in such processes. The discovery of the catalyst composition of the instant invention, therefore, is believed to be a decided advance in the catalyst art.